(1) Field of the Invention
The present invention relates to a method and apparatus for applying a wafer, such as a semiconductor wafer, to a mount plate to be arranged into a polishing machine for polishing the surface of the wafer smooth. Particularly, the present invention relates to a method for applying a wafer to a mount plate so that no air bubbles are occluded between the wafer and the mount plate.
(2) Background Information
Since semiconductor devices have become ultrahighly integrated, wafers used for such semiconductor devices are required to have an extremely high degree of flatness. To obtain such a flatness with high precision, the wafer is adhered by adhesive to a mount plate of a polishing machine in preparation to be polished by the polishing machine. However, when the wafer is adhered to the mount plate, if air bubbles are occluded in the adhesive between the wafer and the mount plate, the wafer is bulged at the portions of the mount plate containing the air bubbles. Thus, when polished, the abrasion amount at the portions of the mount plate which bulge as a result of the air bubbles will be larger than the other portions of the mount plate, and uniform and adequate polishing of the entire surface of the wafer becomes impossible. Accordingly, various methods have been conducted to prevent the occlusion of air bubbles between the wafer and the mount plate.
To carry out the removal of such air bubbles, a method is known wherein a wafer is applied on a mount plate with an adhesive and then the air bubbles which are occluded between the wafer and the mount plate are pressed out. In this method, before the hardening of the adhesive, the wafer is gradually pressed against the mount plate with an elastic member from the central portion of the wafer toward the peripheral portion thereof (Japanese Unexamined Utility Model Publication No. 4-88033). In another method, the area around the wafer is placed in a vacuum condition and then the wafer is pressed against the mount plate. In this method, the area around the wafer is surrounded with a skirt portion and the inside of the skirt portion is placed in a vacuum condition, and then the wafer is pressed against the mount plate in the vacuum atmosphere without the occlusion of air bubbles (Japanese Unexamined Patent Publication No. 64-45567).
The aforementioned conventional methods for applying a wafer to a mount plate include the step of pressing the wafer against the mount plate with an elastic member or the like to remove the air bubbles. Such a step also requires controlling the adjustment of the pressing force and/or the adjustment of the time of application of the pressing force, which are complicated and time consuming. Furthermore, since the wafer is thin, when the wafer is pressed the press mark on the surface of the wafer may sometimes remain as a small concavity, thereby imparting a wavy surface to the wafer.
Particularly, in the method wherein the wafer is held by suction force with a vacuum chuck and is pressed against the mount plate in the vacuum atmosphere by pressing down the vacuum chuck through a piston, the wafer is pressed strongly against the mounting plate and a press mark by the chuck is likely to remain on the surface of the wafer. When polished, the press mark will form a concave portion and prevent uniform and smooth polishing of the entire surface of the wafer. Furthermore, in this conventional method, the wafer is pressed by atmospheric pressure acting on the piston of the vacuum chuck, namely, the pressing is controlled by the relative pressure between the vacuum pressure of the vacuum chuck and the atmospheric pressure. Hence, since the holding force or the pressing force depends on whether there is a low or high atmospheric pressure weather condition, the wafer may drop or the pressing force may become insufficient during operation, leading to adjustment difficulties.